(1) Field of the Invention
The present invention relates to a suspension system for a vehicle and, more particularly, to a suspension system which can improve controllability and stability of the vehicle, ride comfort and the ability of the driver to control the vehicle by automatically controlling a height of a roll center in accordance with a driving state of the vehicle.
(2) Description of Related Art
The primary functions of a suspension system are to (1) provide vertical compliance so wheels can follow an uneven road, thereby isolating a vehicle body from roughness in the road, (2) maintain the wheels in proper steer and camber attitudes to the road surface, (3) react to control forces produced by tires-longitudinal acceleration and braking forces, lateral force, and braking and driving torques, (4) resist roll of the vehicle body, and (5) keep the tires in contact with the road with minimal load variations.
Particularly, rolling occurs on the basis of a predetermined point, called a roll center, when the vehicle is cornering. The variation rate of the height of the roll center has an effect on the running safety and handling of the vehicle.
That is, rolling of the vehicle occurs when the height of a gravitational center of the vehicle body becomes higher than that of the roll center. If the height of the roll center is lowered, it is easy to optimize camber and tread changes such that vibration from the road can be minimized to improve high-speed straight driving performance and safety, as well as ride comfort.
However, when cornering, as lateral force is produced, it becomes difficult to control the vehicle. This acts to diminish the safety of the vehicle.
To solve this problem, it is possible to raise the height of the roll center to increase resistance against rolling such that a shorter anti-roll bar can be used. Doing this, however, enlarges changes of the camber and tread such that the ride comfort and straight running safety are deteriorated.
Accordingly, it is very important to optimally set the height of the roll center by compromising between the two characteristics when designing the suspension system.
Referring to FIG. 7, there is shown a schematic view illustrating a conventional suspension system which is designed to variably control the height of the roll center. The suspension system comprises a wheel carrier 2 rotatably supporting a wheel 1; a strut assembly 3 consisting of a shock absorber 30 and a spring 31 and connecting the upper side of the wheel carrier 2 to a vehicle body 10; and a lower arm 4 connecting the lower side of the wheel carrier 2 to the vehicle body 10.
The suspension system further comprises a hydraulic bushing 50 disposed between the lower arm 4 and the vehicle body 10, a hydraulic source 60 generating hydraulic pressure, a 4-port 3-position center bypass type directional control valve 51 for controlling hydraulic pressure directed to the hydraulic bushing 50, and a control unit 52 for controlling the directional control valve 51 in response to signals from a lateral acceleration sensor 520 and a vehicle speed sensor 521.
FIG. 8 is a sectional view illustrating the hydraulic bushing 50 depicted in FIG. 7. The hydraulic bushing 50 comprises an inner tube 500 connected to the vehicle body 10, an outer tube 501 connected to the lower arm 4, and an elastic member 503 disposed between the inner and outer tubes 500 and 501. Two fluid chambers 504 and 505 connected to the directional control valve 51 through first and second fluid conduits 510 and 511, respectively, are formed within the elastic member 503.
In the above described suspension system, the directional control valve 51 directs hydraulic pressure to one of the chambers 504 and 505 of the elastic member 503 in accordance with a control of the control unit 52 receiving signals from the lateral acceleration and vehicle speed sensors 520 and 521 according to a vehicle's driving condition. At this point, one of the chambers 504 and 505 where the hydraulic pressure is directed expands, while the other of the chambers 504 and 505 where the hydraulic pressure is not directed contracts. As a result, the outer tube 501 connected to the lower arm 4 is displaced relative to the inner tube 500 connected to the vehicle body 10. Namely, the vehicle body side connecting portion of the lower arm 4 is displaced upwardly and downwardly with respect to the vehicle body 10, thereby varying the height of the roll center to improve the running safety and handling of the vehicle.
However, in the above described suspension system, since a large amount of hydraulic pressure is required to expand and retract the elastic member, the expanding chamber is applied with high pressure such that it is easy for the hydraulic bushing to be damaged.
In addition, when the vehicle is in a normal driving condition, the two chambers are supplied with equal levels of hydraulic pressure. However, it is difficult for the directional control valve to maintain the equal levels of hydraulic pressure. For example, in a state where the first conduit 510 is supplied with hydraulic pressure and the second conduit 511 is not supplied with hydraulic pressure, when the directional control valve 51 is displaced into a neutral position, since the first and second conduits 510 and 511 are closed, the pressures within the chambers becomes unequal.
The inherent function of the hydraulic bushing is that it absorbs vibration generated by the wheels. However, if the directional control valve is displaced into the neutral position, since both of the conduits are closed and the level of hydraulic pressure within the chambers is kept, the rigidity of the hydraulic bushing is increased due to the non-compression characteristics of the fluid, deteriorating the vibration absorbing characteristics of the bushing.